Inexpensive sources of purified hydrogen are sought after for many industrial chemical processes and in the production of energy in fuel cell power systems. Similarly, inexpensive methods of purifying hydrogen could significantly expand the applicability of hydrocarbon reforming, reforming reactors and the water gas shift reaction. In order to meet the need for inexpensive purified hydrogen, considerable research effort has been devoted to developing more effective hydrogen permeable gas separation membrane systems which can be used to selectively recover hydrogen from different industrial gas streams containing hydrogen and other molecular components. Hydrogen-permeable membranes made of palladium have been widely studied due to their high hydrogen permeability, and their theoretically infinite hydrogen selectivity. However, one of the problems with palladium membranes, in addition to their high cost, is that they are susceptible to poisoning by hydrogen sulfide which is formed when sulfur sources present in the industrial process gases come in contact with the palladium membrane at a high temperature.
In an attempt to overcome these problems with pure palladium membranes, alloys of palladium have been formulated, such as the alloys of palladium with copper, silver and gold disclosed in U.S. Pat. No. 3,350,845, which were found to have improved resistance to poisoning by hydrogen sulfide. The palladium-gold alloys disclosed in U.S. Pat. No. 3,350,845 were prepared in the form of 1 mil thick foils supported on 1/16″ thick stainless steel substrates. The specific method used to prepare these palladium-gold alloy foils was not disclosed, and apparently involved conventional foil preparation technology. The relatively thick palladium-gold alloy foils disclosed in U.S. Pat. No. 3,350,845 are prohibitively expensive, and would not be suitable for many current industrial applications.
One known method of plating gold is by use of an electrolytic process in which a current is passed through a solution containing a gold salt, such as gold cyanide. The cyanide waste from such a bath presents an environmental problem. To avoid this environmental problem, other gold salts such as gold sulfite and gold thiosulfite have been employed. However, these salts would not be suitable for plating gold on palladium since palladium is poisoned by sulfur compounds.
Another method of plating gold on the surface of a metal involves galvanic displacement using chloroauric acid. In this method, a metal such as nickel, which is more easily oxidized than gold, is electrolessly deposited on the surface on which gold is to be deposited. The freshly deposited metal, e.g., nickel, is readily dissolved by the action of chloroauric acid, which results in the gold ion in solution being reduced, and replacing the surface metal. Galvanic displacement methods using chloroauric acid would not be feasible for depositing gold on palladium membranes to produce sulfur-resistant membranes, because of the high cost of palladium which would be displaced by the gold. Also, palladium membranes are typically annealed and may be polished, which provides them with a smooth surface that is difficult to plate.
Chloroauric acid has also been used to deposit gold films onto other surfaces such as glass. For example, a paper by Jiandong Hu et al, entitled “Novel Plating solution for electroless plating of gold film onto glass surface”, Surface and Coatings Technology (2008), 202 (13), 2922-2926, published by Elsevier, describes an electroless gold plating process in which chloroauric acid and hydrogen peroxide were used to deposit a gold film onto (3-aminopropyl)-trimethoxysilane-coated glass. The Jiandong Hu et al paper teaches that in order to achieve the deposition of gold onto insulating substrates, the inert surface must be activated or functionalized with the use of silanizing agents, such as (3-aminopropyl)-trimethoxysilane (APTMS) to enhance the gold adhesion to the substrate. The method of preparation of the surface of the glass was quite extensive and involved a cleaning treatment to remove surface contamination, an oxidation treatment with piranha solution to form an oxygenated species on the glass surface, and a modifying treatment with a silanizing reagent to such as APTMS to form the modified glass surface. The multi-step method disclosed in this paper was employed to prepare gold coated glass slides. There is no disclosure in the paper regarding the preparation of gas separation membranes.
A more recent approach to preparing a sulfur-resistant palladium-gold membrane is described in WO 2008/027646, in which a sulfur-resistant composite palladium alloy membrane was prepared by seeding a porous substrate with palladium crystallites, decomposing any organic liquids present on the substrate, reducing the palladium crystallites on the substrate to the metallic form, depositing a film of palladium metal on the substrate and then depositing a second, gold film on the palladium film. The deposition of the gold film was accomplished by pumping a solution containing water, NaOH and gold (III) chloride over the surface of the substrate.
While the method disclosed in WO 2008/027646 involving seeding with palladium crystallites and use of a NaOH and gold (III) chloride plating solution, provides a sulfur-resistant composite gas separation membrane, there is a continuing need in the art for fabrication methods capable of inexpensively and efficiently producing gas separation membrane systems which are resistant to poisoning by sulfur.
The present invention provides an inexpensive, highly efficient, and environmentally friendly method for preparing a sulfur-resistant, thermally stable, gold-palladium alloy gas separation membrane system. This method does not involve galvanic displacement, does not require the use of palladium crystallites, and does not require any expensive activation, cleaning or functionalization techniques.